A magnetic induction flow meter is used as a flow meter for liquids, pulps and pastes which have a specific minimum electric conductivity. This type of flowmeter is distinguished by quite accurate measured results, no pressure loss in the pipeline system being caused by the measurement. In addition, magnetic induction flowmeters have no parts which move or project into the measuring tube and which, in particular, are subject to wear. The area of use of the flowmeters of interest here extends primarily to applications in the chemical industry, pharmaceuticals and the cosmetic industry, and also the communal water and effluent business as well as the foodstuffs industry.
Faraday's induction law forms a physical basis of the measuring method of a magnetic induction flowmeter. This natural law states that a voltage is induced in a conductor moving in a magnetic field. During the utilization of this natural law for measurement, the electrically conductive medium flows through a measuring tube in which a magnetic field is generated at right angles to the flow direction. The voltage induced in the medium is picked up by an electrode arrangement. Since the measuring voltage obtained in this way is proportional to the average flow velocity of the flowing medium, the volume flow of the medium can be determined from it. In addition, by taking account of the density of the flowing medium, its mass flow can also be determined.
Measuring tubes of such magnetic induction flowmeters are primarily composed of metal and, in order to implement the measuring principle, are provided with internal electrical insulation. In addition, there also exist plastic measuring tubes, in which it is usually possible to dispense with additional internal electrical insulation. The present disclosure relates to the last-named type of measuring tubes.
DE 103 47 890 A1 reveals a generic magnetic induction flowmeter whose measuring tube is composed of plastic. In order to produce the measuring tube, a fiber composite material is used. The measuring electrodes that contact the substance to be measured are composed of metal plates, which are placed on a core and are subsequently enveloped by the first wound layers of the fiber composite material. After the wound layers have been cured, the core is removed, so that the measuring electrodes are given contact with the conductive flow medium. The surface structure of the measuring tube can be configured within wide limits because of the winding technique. For example, molded-on portions can be produced in the end region of the measuring tube in order to achieve the largest possible sealing areas. The magnetic unit is preferably embedded completely within the wall of the measuring tube and is therefore accommodated therein in a protected manner.
The wall thickness of the measuring tube consisting of plastic has to be dimensioned in such a way that the material withstands the external mechanical loadings. Disruptive external mechanical loadings are produced, for example, by the pipeline into which the measuring tube is inserted. Thus, depending on the installation situation, disruptive mechanical influences in the form of axial forces and/or thrust forces can occur on the measuring tube and, in the extreme case, can lead to cracking or even to fracture of the measuring tube.
Although these problems can be counteracted by an appropriate material selection and also by generous dimensioning of the wall thickness, this is often associated with considerable expenditure on material and fabrication.